As a plate glass used as a window glass of a vehicle, adopted is one which has a complex three-dimensional shape in accordance with the requirements such as the design and the aerodynamic characteristics. When seeing an object through such a window glass constituted of a three-dimensional curved surface, the object may be seen as distorted. This phenomenon is called a perspective distortion phenomenon and it is known that the phenomenon occurs in a nonparallel portion and a curved portion of a plate glass. The perspective distortion is a disincentive for the visibility of a driver especially when driving a vehicle, and therefore, the permissible maximum value of the perspective distortion should be determined. Currently, although whether or not the perspective distortion falls within the permissible maximum value is inspected before product shipment, the perspective distortion of a glass is visually inspected by an inspector. Therefore, there are problems that the visual inspection varies between individuals, and the productivity cannot be enhanced due to inspection inefficiency.
As the prior art, in Patent Document 1, disclosed is a method in which the perspective distortion of an object to be inspected is inspected by capturing an LED plate which is disposed in front of the object to be inspected and constituted of a plurality of segments forming a predetermined angle, by a camera which is disposed behind the object to be inspected, through the object to be inspected. In this method, a reference length of each segment was measured in such a state that the object to be inspected was removed, and the reference length and a length with distortion which is measured through the object to be inspected are compared with each other.    Patent Document 1: Japanese Patent Application Laid-Open No. 6-144006